Perpendicular sttmram device with balanced reference layer

ABSTRACT

A spin transfer torque magnetic random access memory (STTMRAM) element comprises a reference layer, which can be a single layer structure or a synthetic multi-layer structure, formed on a substrate, with a fixed perpendicular magnetic component. A junction layer is formed on top of the reference layer and a free layer is formed on top of the junction layer with a perpendicular magnetic orientation, at substantially its center of the free layer and switchable. A tuning layer is formed on top of the free layer and a fixed layer is formed on top of the tuning layer, the fixed layer has a fixed perpendicular magnetic component opposite to that of the reference layer. The magnetic orientation of the free layer switches relative to that of the reference layer. The perpendicular magnetic components of the fixed layer and the reference layer substantially cancel each other and the free layer has an in-plane edge magnetization field.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of the commonly assignedapplication bearing Ser. No. 14/026,163 filed on Sep. 13, 2013 andentitled “Perpendicular STTMRAM Device with Balanced Reference Layer,”which is a continuation-in-part of the commonly assigned applicationbearing Ser. No. 13/029,054 filed on Feb. 16, 2011 by Zhou et al. andentitled “Magnetic Random Access Memory With Field Compensating Layerand Multi-Level Cell,” and a continuation-in-part of the commonlyassigned application bearing Ser. No. 13/277,187 filed on Oct. 19, 2011by Huai et al., and entitled “Memory System Having Thermally StablePerpendicular Magneto Tunnel Junction (MTJ) and A Method ofManufacturing Same,” which claims priority to U.S. ProvisionalApplication No. 61/483,314. The present application is related to thecommonly assigned copending application bearing Ser. No. 13/737,897filed on Jan. 9, 2013, the commonly assigned copending applicationbearing Ser. No. 14/021,917 filed on Sep. 9, 2013, the commonly assignedcopending application bearing Ser. No. 13/099,321 filed on May 2, 2011,and the commonly assigned copending application bearing Ser. No.13/928,263.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to magnetic memory elementshaving magnetic tunnel junctions (MTJ) and particularly to improving theease of switching of the free layer of the MTJ to reduce the requisitevoltage and current for causing the free layer to switch magneticstates.

2. Description of the Prior Art

Magnetic random access memory (MRAM) is rapidly gaining popularity asits use in replacing conventional memory is showing promise. Magnetictunnel junctions (MTJs), which are essentially the part of the MRAM thatstore information, include various layers that determine the magneticbehavior of the device. An exemplary MTJ uses spin transfer torque toeffectuate a change in the direction of magnetization of one or morefree layers in the MTJ. That is, writing bits of information is achievedby using a spin polarized current flowing through the MTJ, instead ofusing a magnetic field, to change states or program/write/erase/readbits.

In spin transfer torque (STT) MTJ designs, when electrons flow acrossthe MTJ stack in a direction that is perpendicular to the film plane orfrom the pinned (sometimes referred to as “reference” or “fixed”) layerto the free (or storage) layer, spin torque from electrons transmittedfrom the pinned layer to the free layer orientates the free layermagnetization in a direction that is parallel to that of the referenceor pinned layer. When electrons flow from the free layer to the pinnedlayer, spin torque from electrons that are reflected from the pinnedlayer back into the free layer orientates the free layer magnetizationto be anti-parallel relative to the magnetization of the pinned layer.Thus, controlling the electron (current) flow direction, direction ofmagnetization of the free layer magnetization is switched. Resistanceacross the MTJ stack changes between low and high states when the freelayer magnetization is parallel or anti-parallel relative to that of thepinned layer.

However, a problem that is consistently experienced and that preventsadvancement of the use of MTJs is the threshold voltage or current usedto switch the free layer magnetization during a write. Such current andthreshold voltage requirements are currently too high to allow practicalapplications of the spin transfer torque based MTJ.

MTJs with perpendicular anisotropy, such that the magnetic moments ofthe free layer and the fixed layer thereof are in perpendiculardirections relative to the planes of the films, are more appealing thantheir in-plane anisotropy counterparts largely due to the densityimprovements realized by the former. Existing perpendicular MTJ designsinclude a free layer whose magnetic orientation relative to a reference(“fixed”) layer, while perpendicular in direction, high coercivity field(Hc) of the free layer, at its edges, limits the reduction of theeffective Hc of the free layer. Lower effective Hc of the free layerwould allow easier switching of the free layer and would lower thethreshold voltage and current required to switch the magnetization ofthe free layer.

It is noted that the foregoing problem occurs due to the inconsistent Hcthroughout the free layer, as shown and discussed by way of a graphshortly. That is, perpendicular anisotropic field (Hk) of the free layerchanges relative to the position within the free layer such that thecenter of the free layer generally has a lower Hc than the outer edgesof the free layer with Hc essentially increasing from the center of thefree layer to its outer edges. Accordingly, efforts to lower theperpendicular anisotropic field (Hk) of the free layer in order to easeswitching result in lowering of effective Hc, undesirably increase theedge-to-center effective coercivity (Hc) ratio. The relationship betweenHk and Hc is as follows:

Hc=Hk−Hdemag   Eq. (1)

where Hdemag is the demagnetization field related to the magneticmoment, thickness, shape and size of the magnetic thin film.

For a greater understanding of the foregoing problem, FIGS. 1-3 show arelevant portion of a prior art magnetic memory element and a graph ofits effective coercivity field performance.

FIG. 1 shows the relevant portion of a prior art magnetic random accessmemory (MRAM) element 10, which includes a reference layer 3, also knownas a fixed layer, a barrier layer 2, also known as a tunnel layer, and afree layer 1. This configuration is common and very well known in theart. The layers 1-3 are often times collectively referred to as amagneto-tunnel junction (MTJ). When an electron current is appliedthrough the layer 3 towards layer 1, for example during a writeoperation, the MRAM element 10 switches states where the magnetic momentof the layer 1 changes direction relative to the magnetic moment of thelayer 3, from a direction shown by the arrow 5 to a direction shown bythe arrow 6. Such a change in the layer 1 is also known as a change froman anti-parallel state, where the direction of the magnetic moment ofthe layer 1 is opposed to that of the layer 3 to a parallel state, wherethe direction of the magnetic moment of the layer 1 is same as that ofthe layer 3. The resistance of the MRAM element 10 changes according toits state and typically, such resistance is higher when the MRAM is inan anti-parallel state than when it is in a parallel state.

Lowering the perpendicular Hk of the layer 1 would make switching of thestate of the MRAM 10 easier, however, as earlier noted, the effective Hcreduction, which would significantly ease switching of the state of theelement 10 is limited because of the high Hc present at the edges of thelayer 1. This is better noticed by the figures to follow.

FIG. 2 shows generally a top view 7 of the layer 1 of FIG. 2 and a sideview 8 of the layer 1 of FIG. 2. The layer 1 is shown to be 65 nanometers in diameter, by way of example, and 1.2 nano meters in thickness.In accordance with these measurements, the effective Hc, in kiloOersteds, vs. the position along the diameter of the layer 1, in nanometers (nm), is shown in a graph in FIG. 3. Accordingly, FIG. 3 shows agraph of the effective Hc, represented by the y-axis, vs. the positionalong the diameter of the layer 1, represented by the x-axis, for thecase where the perpendicular Hk (p-Hk) is equal to 14.5 kilo Oersteds(kOe), shown by a line 10 and for the case where the perpendicular Hk ofthe layer 1 is equal to 13 kOe, shown by a line 11. As shown, theeffective Hc increases going from the center of the layer 1 out to itsedge and this change gradually increases at the far edge of the layer 1.When decreasing the perpendicular Hk from 14.5 kOe to 13 kOe, theedge-to-center effective Hc ratio is undesirably increased from 1.6 to3.0.

Thus, the need arises for decreasing the perpendicular anisotropic fieldof the free layer of an MRAM yet avoiding a substantial increase in theeffective Hc of the MRAM in order to reduce the threshold voltage andcurrent required to operate the MRAM.

SUMMARY OF THE INVENTION

Briefly, a spin transfer torque magnetic random access memory (STTMRAM)element is disclosed for storing a state when electrical current isapplied to it. The STTMRAM element includes a reference layer, formed ona substrate, having a perpendicular magnetic component associatedtherewith that is fixed in one direction. A junction layer is formed ontop of the reference layer and a free layer is formed on top of thejunction layer and has a magnetic orientation, at substantially thecenter of it that is perpendicular relative to the substrate andparallel and switchable relative to the reference layer. Further, aspacer layer is formed on top of the free layer and a fixed layer isformed on top of the spacer layer, the fixed layer having aperpendicular magnetic component associated therewith that is fixed in adirection opposite to that of the reference layer. The free layer iscapable of switching its magnetic orientation relative to the fixedlayer when electrical current is applied to the STTMRAM element. Theperpendicular magnetic components of the fixed layer and the referencelayer substantially cancel each other and the free layer has amagnetization field at its edge that is in-plane relative to thesubstrate.

According to another aspect of the present invention, an STTMRAM elementincludes a magnetic pinned layer having a first fixed magnetizationdirection substantially perpendicular to the film plane thereof, amagnetic free layer separated from the magnetic pinned layer by anon-magnetic tuning layer and having a variable magnetization directionsubstantially perpendicular to the film plane thereof, and a magneticreference layer separated from the magnetic free layer by an insulatingtunnel junction layer and having a second fixed magnetization directionsubstantially opposite to the first fixed magnetization direction of themagnetic pinned layer. The magnetic pinned layer and the magneticreference layer have different magnetic switching fields and are formedon opposite sides of the magnetic free layer. The tuning layer has athickness that allows the offset field in the magnetic free layer asexerted by the magnetic pinned layer and the magnetic reference layer tobe about zero. The magnetic reference layer may further comprise a firstand second magnetic reference layers interposed by a coupling layertherebetween.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the various embodiments illustrated inthe several figures of the drawing.

IN THE DRAWINGS

FIG. 1 shows the relevant portion of a prior art magnetic random accessmemory (MRAM) element 10, which includes a reference layer 3, also knownas a fixed layer, a barrier layer 2, also known as a tunnel layer, and afree layer 1.

FIG. 2 shows generally a top view 7 of the layer 1 of FIG. 2 and a sideview 8 of the layer 1 of FIG. 2.

FIG. 3 shows a graph of the effective Hc, represented by the y-axis, vs.the position along the diameter of the layer 1, represented by thex-axis, for the case where the perpendicular Hk (p-Hk) is equal to 14.5kilo Oersteds (kOe), shown by a line 10 and for the case where theperpendicular Hk of the layer 1 is equal to 13 kOe, shown by a line 11.

FIG. 4 shows the relevant portion of a spin transfer torque magneticrandom access memory (STTMRAM) element 30, in accordance with anembodiment of the present invention.

FIG. 5 shows a top view 35 of the layer 21 and a side view 37 of thelayer 21, in accordance with an embodiment of the present invention.

FIG. 6 shows the relevant magnetization fields of the layers 23, 22, 21,24 and 25 of the element 30, in accordance with an embodiment of thepresent invention.

FIG. 7 shows a graph 47 of the performance of the element 30, inaccordance with an embodiment of the present invention.

FIG. 8 shows a graph of the normalized switching voltage of the element30 as the in-plane magnetic edge field of its layer 21 increases.

FIG. 9 shows a graph of the performance of the element 30 when variouslevels of edge fields, including none, are applied to the layer 21 ofthe element 30.

FIG. 10 shows the relevant portion of a spin transfer torque magneticrandom access memory (STTMRAM) stack 55, in accordance with anembodiment of the present invention.

FIG. 11 shows the formation of the stack 55, in relevant part andaccordance with a method of the present invention, as two steps.

FIG. 12 shows the formation of the stack 30, in relevant part andaccordance with another method of the present invention, as two steps.

FIG. 13 shows an STTMRAM element according to an embodiment of thepresent invention.

FIG. 14 shows TMR dependence curves on annealing temperature for STTMRAMelements 10 and 100.

FIG. 15 shows an STTMRAM element according to another embodiment of thepresent invention.

FIG. 16A shows dependence of offset field in the free layer on thetuning layer thickness for various pinning layer magnetic moment andthickness product of the STTMRAM element 500, in accordance with anembodiment of the present invention.

FIG. 16B shows dependence of offset field in the free layer on thetuning layer thickness for various reference layer magnetic moment andthickness product of the STTMRAM element 500, in accordance with anotherembodiment of the present invention.

FIG. 17 shows the free layer portion of an STTMRAM element 500 inaccordance with an embodiment of the present invention.

FIG. 18 shows the free layer portion of an STTMRAM element 500 inaccordance with another embodiment of the present invention.

FIG. 19 shows the free layer portion of an STTMRAM element 500 inaccordance with still another embodiment of the present invention.

FIG. 20 shows the reference layer portion of an STTMRAM element 500 inaccordance with an embodiment of the present invention.

FIG. 21 shows the reference layer portion of an STTMRAM element 500 inaccordance with another embodiment of the present invention.

FIG. 22 shows the reference layer portion of an STTMRAM element 500 inaccordance with still another embodiment of the present invention.

FIG. 23 shows TMR dependence curves on annealing temperature for STTMRAMelements 10, 100, and 500.

FIG. 24 shows an STTMRAM element according to still another embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of the specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized because structural changes may be madewithout departing from the scope of the present invention. It should benoted that the figures discussed herein are not drawn to scale andthicknesses of lines are not indicative of actual sizes.

In an embodiment of the present invention, a spin transfer torquemagnetic random access memory (STTMRAM) element and a method ofmanufacturing the same is disclosed. Relevant layers of the STTMRAMelement include a reference layer, formed on a substrate, with aperpendicular magnetic component that is fixed in one direction. Ajunction layer is formed on top of the reference layer and a free layeris formed on top of the junction layer and has a magnetic orientation,at substantially the center of it that is perpendicular relative to thesubstrate and parallel and switchable relative to the reference layer.Further, a spacer layer is formed on top of the free layer and a fixedlayer is formed on top of the spacer layer, the fixed layer having aperpendicular magnetic component associated therewith that is fixed in adirection opposite to that of the reference layer. The free layer iscapable of switching its magnetic orientation relative to the fixedlayer when electrical current is applied to the STTMRAM element. Theperpendicular magnetic components of the fixed layer and the referencelayer substantially cancel each other and the free layer has amagnetization field at its edge that is in-plane relative to thesubstrate.

In an alternative embodiment, a stack is formed of multiple STTMRAMelements where each element is formed on top of another element allowingthe stack to store more than one state. FIG. 4 shows the relevantportion of a spin transfer torque magnetic random access memory(STTMRAM) element 30, in accordance with an embodiment of the presentinvention. The STTMRAM element 30 is shown to include a reference layer23, sometimes referred to as a “fixed layer”, on top of which is formeda junction layer 22, sometimes referred to as a “barrier layer” or a“tunnel layer” or a “barrier tunnel layer” or a “tunnel barrier layer”,on top of which is formed a free layer 21, sometimes referred to as a“switching layer”, on top of which is formed a separator layer 24,sometimes referred to as a “spacer layer”, on top of which is formed afixed layer 25, sometimes referred to as a “reference layer”. It isunderstood that other layers, not shown in FIG. 4, may be and aretypically formed on top of the layer 25, below the layer 23 and/or inbetween any of the layers shown in FIG. 4.

The layer 23 is shown to have a magnetic moment (also known as magneticorientation) in a direction shown by the arrow 231, the layer 21 isshown to have a magnetic moment in a direction shown by the arrow 211and the layer 25 is shown to have a magnetic moment in a direction shownby the arrow 251. An electrical current is applied, either at 31 or at32, to the element 30 during read and write operations. The element 30is generally used to store digital information during write or programoperations and this information is read during read operations. Forthese operations, various devices are coupled thereto that are not shownin FIG. 4. For a description of these devices and methods of readingfrom and writing to the element 30, the reader is directed to U.S.patent application Ser. No. 11/674,124, filed on Feb. 12, 2007, by RajivYadav Ranjan, and entitled “Non-Uniform Switching Based Non-VolatileMagnetic Based Memory”, the disclosures of which are incorporated hereinby reference.

The element 30 has a perpendicular anisotropy in that the layer 21 has amagnetic moment that is perpendicular relative to that of the film ontop of which the element 30 is formed. Similarly, the layers 25 and 23have such a perpendicular anisotropy. The element 30 switches states andstores a digital value corresponding to the magnetic orientation of thelayer 21 in that when this orientation is parallel to the magneticorientation of the layer 23, the element 30 is in one state, generallyreferred to as “parallel”, and when the orientation of the layer 21 isnot parallel, or anti-parallel, relative to the orientation of the layer23, the element 30 is in another state. These different states result inunique resistances across the MRAM stack. In this manner, the digitalvalue of ‘1’ or ‘0’ is distinguished during read/write operations.

The layers 23 and 25 are made generally of material that is known in theart to be used for such fixed layer. Similarly, the layer 21 is made ofmaterial typically used by the industry for making a free layer, as isthe layer 22 made of material known for making a tunnel layer. The layer24, in some embodiments, is a multi-layer, made of at least oneinsulating layer 33, shown in the exploded view of the layer 24 at theleft side of FIG. 4, and at least one conductive layer 34. While one ofeach of the insulating and conductive layers is shown, it is understoodthat ‘n’ number of such a configuration may comprise the layer 24, with‘n’ being an integer value. However, the conductive and insulatinglayers alternate such that no two conductive layers are adjacent to eachother and no two insulating layers are adjacent to each other. Theinsulating layer is made of an insulating material, such as but notlimited to any of the following: alumina (Al₂O₃), magnesium oxide (MgO),silicon dioxide (SiO₂), and oxide of other metallic material, and theconductive layer is made of a conductive material, such as any of thefollowing: ruthenium (Ru), tantalum (Ta), copper (Cu), silver (Ag), gold(Au) and any other metallic non-magnetic element or alloy. Thus, theconductive layer is non-magnetic. In alternative embodiments, theinsulating layer 33 is formed on top of the conductive layer 34.

In some embodiments, the layer 24 is a single layer made of anon-magnetic material. The make-up of the layer 24, coupled with theconfiguration of the element 30, particularly using the layer 25 on topof the layer 24, as shown, cause the presence of magnetic fields atsubstantially the outer perimeter (edges) of the layer 21 with each suchmagnetic field having an in-plane magnetic orientation. These in-planemagnetic fields at the outer edge of the layer 21 effectively reduce theeffect of the high perpendicular Hc, which, as previously discussed,prevents the free layer to readily switch magnetization states in priorart magnetic memories. Accordingly, switching between the states of theelement 30 is eased and in this respect requires lower threshold voltageand current. As is shown in FIG. 7 herein, approximately 70% reductionin voltage is realized by the embodiment of FIG. 4 over that of priorart structures.

FIG. 5 shows a top view 35 of the layer 21 and a side view 37 of thelayer 21, in accordance with an embodiment of the present invention. Asshown, in-plane magnetization field 36, appearing at the edges of thelayer 21 are present despite the perpendicular magnetization field 38,at substantially the center of the layer 21 that is switchable forstoring purposes. The field 36 helps to reduce the effect of highperpendicular He at the edge of the layer 21, thereby causing the layer21 to switch with more ease.

FIG. 6 shows the relevant magnetization fields of the layer 23, 22, 21,24 and 25 of the element 30, in accordance with an embodiment of thepresent invention. The perpendicular magnetic fields, appearingsubstantially at the center, of the layers 23, 22, 24 and 25, are eachshown in a direction consistent with the arrows 45, 43, 44 and 46,respectively. Magnetization directions of layer 23 and layer 25 areperpendicular relative to the plane of the substrate and oppositerelative to each other and not necessarily required to follow thedepicted directions in FIG. 6 and in alternative embodiments are inreverse directions than that shown in FIG. 6.

The magnetization of the layer 25 creates the field shown by the arrow41 and the magnetization of the layer 23 creates the field shown by thearrow 42. The layer 23 is also shown to have significant in-planemagnetic components or fields 40, at its edges, in a direction shown bythe arrows associated with the fields 40. Similarly, the layer 25 isshown to have significant in-plane magnetic fields 39, at its edges, ina direction shown by the arrows of the fields 39. In the case of thelayer 21, the magnetic field of the layer 25 imposes onto the layer 21and is in large part perpendicular at substantially the center of thelayer 21 and the layer 23, as shown by the arrow 44, which extendsthrough the layer 24 such that the perpendicular magnetic field of thelayer 24 is substantially the same as that of the layer 25. Similarly,the magnetic field of the layer 23 onto the layer 21 is in large partperpendicular at substantially the center of the layer 21, as shown bythe arrow 43, which extends through the layer 22 such that theperpendicular magnetic field of the layer 22 is the same as that of thelayer 23.

The perpendicular magnetic fields of the layers 25 and 23 essentiallycancel each other while the in-plane magnetic fields at the edges of thelayer 21, fields 39 and 40, enhance each other and therefore reduce theeffective Hc that would typically be present at the edges of prior artstructures. Accordingly, not only does the process of switching statesbecomes easier and requires less voltage and current, switching is alsoadvantageously substantially symmetrical.

Furthermore, by optimizing the spacing between the layers 25 and 21, themagnetic moment of the layers 23 and 25, the layer 21 can be made toswitch at different voltages due to a difference in the magnitude ofin-plane edge fields in the layer 21. For a greater understanding of theeffect of the edge field on the switching voltage, a graph is shown anddiscussed relative to FIG. 7 and FIG. 8. By increasing the thickness oflayer 24, layer 25 is further separated from the layer 21, therebycreating a weaker magnetic field in layer 21 than when the thickness oflayer 24 is not so increased. To still fully compensate for the verticalfield from layer 23 into the layer 21, magnetic moment of layer 25 canincrease. While the vertical field is compensated, the edge in-planefield from the layer 25 onto layer 21 is lower than before as thespacing between layer 21 and layer 25 increases. Thus, with a lower edgein-plane field in layer 21, layer 21 would be harder to switch thanbefore.

FIG. 7 shows a graph 47 of the performance of the element 30 vs. aconventional MRAM structure as in FIG. 1, in accordance with anembodiment of the present invention. Graph 47 is shown to have a y-axisindicative of the normalized resistance, in arbitrary unit (a.u.) vs. anx-axis indicative of the normalized switching voltage, in a.u., of thevoltage required to switch the free layer in an MRAM structure fromanti-parallel to reference layer, i.e. high resistance, state toparallel, i.e. low resistance, state, where the drop in the curvesmarked the switching voltage of the free layer. The dashed line shown at48 is the performance of a conventional magnetic random access memoryelement as in FIG. 1 and the line, shown at 49, with circles thereon isthe performance of the element 30. As shown by the graph 47, the element30 exhibits far lower switching voltage than its counterpart structure.A reduction of more than 70% in the switching voltage is realized.

FIG. 8 shows a graph of the normalized switching voltage of the element30 as the in-plane magnetic edge field of its layer 21 increases. Thegraph of FIG. 8 has a y-axis that represents the normalized switchingvoltage of the element 30 in a.u. and an x-axis that represents thein-plane edge magnetic field of the layer 21, in kOe. As shown, theswitching voltage reduces faster at higher in-plane edge magnetic fieldof the element 30. For example, at the normalized switching voltage of10, the edge field is approximately zero whereas at the normalizedswitching voltage of approximately 5.5, the edge field is approximately4. Thus, by controlling the edge field, the layer 21 can be made toswitch states at different voltages.

FIG. 9 shows a graph of the performance of the element 30 when variousstrengths of edge field, including none, are applied to the layer 21 ofthe element 30. The graph of FIG. 9 includes a y-axis representing thenormalized resistance, in a.u., of the element 30 relative to thenormalized voltage, in a.u., of the element 30, shown by the x-axis,when there is no in-plane edge field applied, shown at 50, and when anin-plane edge field of 1 kOe is applied, as shown at 51, and when anin-plane edge field of 2 kOe is applied, as shown at 52, and when anin-plane edge field of 3 kOe is applied, as shown at 53, and when anin-plane edge field of 5 kOe is applied, as shown at 54. As shown, withthe applied in-plane edge field increasing from none to 5 kOe, thevoltage required to switch the layer 21 advantageously decreases from10.4 to 3.2 which is approximately a 70% reduction.

FIG. 10 shows the relevant portion of a spin transfer torque magneticrandom access memory (STTMRAM) stack 55, in accordance with anembodiment of the present invention. The stack 55 is shown to includethe layers of the element 30 and on top of the layer 25 thereof is shownformed a spacer layer 34 on top of which is shown formed free layer 31on top of which is shown formed junction layer 32 on top of which isshown formed reference layer 33. The layers 21-23 form an MTJ 56 and thelayers 31, 32 and 33 form an MTJ 57. Thus, MTJs 56 and 57 are stacked.Because two MTJs form the stack 55, the stack 55 is capable of storingtwo states. It is understood that the stack 55 may employ any number ofMTJs and clearly, the more MTJs employed, the greater the number ofstates that can be stored in the stack 55. Thus, the stack 55 isconsidered to be a multi-state element.

Similar to the layer 24, the layer 34 is non-magnetic, in one embodimentof the present invention, and is accordingly made of an insulating layeror a conductive layer. In other embodiments, the layer 34, again similarto the layer 24, is multi-layered and made of any number of alternatingoxide and conductive layers. The layers 31, 32 and 33 are made ofmaterial analogous to that of the layers 21-23, respectively. In someembodiments, the thicknesses of the layers 31, 32 and 33 are analogousto those of the layers 21-23, respectively, and in alternativeembodiments, the thicknesses of the layers 31, 32 and 33 are differentthan those of the layers 21-23, respectively. The layers 23 and 33 havedifferent magnetic moments in some embodiments, and similar magneticmoments in other embodiments. Different moments cause different fieldsin the respective free layers and thus different edge fields anddifferent switching voltages associated with each of the free layers,even when the free layers are identical in material and/or thickness.The layers 24 and 34 each has a different thickness relative to theother. The effective in-plane edge magnetic field of the layer 21, asproduced by the layers 23 and 25, is different than the effectivein-plane edge magnetic field of the layer 31, which is produced by thelayers 25 and 33. This is largely due to the requirement of each of thelayers 21 and 31 having a unique current density to switch, as known tothose skilled in the art. That is, briefly, the MTJs of the stack 55cause it to be a multi-state element where each MTJ's unique switchingcurrent density results in a different state being programmed to from amulti-level cell. Accordingly, the effective in-plane magnetic edgefield of each of the MTJs must also be at a different strength.

Layer 25 in FIG. 10 is a single magnetic layer in some embodiments and amagnetic multilayer structure with magnetic layers interleaved bynon-magnetic layer in other embodiments. Such non-magnetic layer can bemetallic, or metal oxide, or interlacing of both.

FIG. 11 shows the formation of the stack 55, in relevant part andaccordance with a method of the present invention, as two steps. Afterthe formation of the layers of the stack 55, field 60 is applied to thestack 55 to magnetize the layers 23, 33 and 25 such that the directionof magnetization at the center of each of these layers is substantiallypointing in the same direction as the direction of magnetization at thecenter of the rest of these layers, as shown by the arrows 231, 331, and251, respectively. Also, the magnetic orientation at the center of eachof the layers 23 and 25 and 33 is parallel relative to that of theothers. Field 60 has an orientation consistent with the direction of thearrow shown at field 60. This completes Step 1 but in thisconfiguration, clearly, the magnetic fields at the center of the layers25 and 23 do not cancel each other. Thus, next, at Step 2, field 61 isapplied to the stack 55. The direction of the magnetic orientation offield 61 is consistent with the direction of the arrow shown at field61, which is opposite to that of field 60. Field 61 is lower in strengththan field 60 and only magnetizes the layer 25 to a direction that isopposite to that of the layers 23 and 33. In other embodiments, field61, while still lower than field 60, only magnetizes the layers 23 and33 such that these layers' magnetization orientation becomes opposite tothat of the layer 25, which is shown at Step 3. Thus, after Step 1,either Step 2 is performed or Step 3 is performed. It is noted that theforegoing method of forming the stack 55 can also be applied to theelement 30 when it is being manufactured.

In accordance with another method of forming the stack 55 and/or theelement 30, the field 60 is applied while the layers of thestack/element are being formed, during the MTJ deposition and annealing,readily known in the art. A temperature of greater than 200 degreesCelsius during the annealing of the MTJ can be used during such aprocess.

FIG. 12 shows the formation of the stack 30, in relevant part andaccordance with another method of the present invention, as two steps.After the formation of the layers of the stack 30, as described relativeto FIG. 4, at step 1, field 70 is applied to the stack 30 to magnetizethe layers 23 and 25 such that the direction of magnetization of each ofthese layers is substantially pointing in the same direction as shown bythe arrows 231, and 251, respectively. At this point, the process eithercontinues to Step 2 or to Step 3. Assuming Step 2 is performed next,field 72 is applied to the stack 30. The direction of the magneticorientation of field 72 is consistent with the direction of theassociated arrows, which is opposite to that of field 70. Field 72 islower in strength than field 70 and in some embodiments only magnetizesthe layer 23 in the direction shown and in other embodiments magnetizesthe layer 25 in a direction substantially opposite to that of the layer23.

Alternatively, after Step 1, Step 3 is performed in a manner analogousto Step 2 except that the layer 25 has a magnetization direction that isopposite to that which it took on at Step 1 but remains opposite to thatof the layer 23 because the layer 23 is magnetized, in Step 3, in thesame direction as that which it took on at Step 1. At step 3, field 74is applied to the element 30 to effectuate the foregoing magnetizations.The direction of field 74, as shown, dictates the directions ofmagnetizations of the layers 23 and 25, at Step 3.

In accordance with another method of forming the element 30, the field70 is applied while the layers of the stack/element are being formed,during the MTJ deposition and annealing, readily known in the art. Atemperature of greater than 200 degrees Celsius during the annealing ofthe MTJ can be used during such a process. The fields 72 and 74 areapplied to the element 30 after the formation of the latter.

It should be noted that in order to achieve the anti-parallelorientation of the magnetizations 231 and 251 of the reference layer 23and the fixed layer 25, respectively, as shown in FIG. 4 using theprocesses described in FIG. 12, the fixed layer 25 and reference layer23 need to have different switching fields of their magnetizations.Thus, magnetizations 231 and 251 of the reference layer 23 and the fixedlayer 25, respectively, can be switched to the same direction with anapplied high field. With an application of a lower field in between theswitching fields of the reference layer 23 and the fixed layer 25 in theopposite direction, magnetization of only one of the reference layer 23and the fixed layer 25 may be switched to the opposite direction toattain an antiparallel orientation of the magnetizations 231 and 251.

FIG. 13 illustrates an STTMRAM element 100 in accordance with anotherembodiment of the present invention. The element 100 includes anon-magnetic seed layer 101, a magnetic free layer 102 formed on top ofthe non-magnetic seed layer 101, a magnetic reference layer 109separated from the magnetic free layer 102 by an insulating tunneljunction layer 103, a magnetic pinned layer 108 exchange coupled to themagnetic reference layer 109 through a Ru layer 107. The magneticreference layer 109 comprises a magnetic interface reference layer 104made of a CoFeB alloy and a magnetic top reference layer 106 formedthereabove with a Ta layer 105 interposed therebetween. The magnetic topreference layer 106 and the magnetic pinned layer 108 may have a facecentered cubic (FCC) lattice structure, while the magnetic interfacereference layer 104 and the magnetic free layer 102 may have a bodycentered cubic (BCC) lattice structure. The insulating tunnel junctionlayer may have a cubic lattice structure.

The magnetic free layer 102 of the element 100 is analogues to the freelayer 1 of the element 10 and is typically made of a CoFeB alloy or aCoFeB based alloy. The magnetic interface reference layer 104 of theelement 100 is analogous to the reference layer 3 of the element 10 andis typically made of a CoFeB alloy or a CoFeB based alloy. The tunneljunction layer 103 of the element 100 is analogous to the junction layer2 of the element 10 and is typically made of a material comprisingmagnesium oxide (MgO).

Post-deposition annealing of the elements 10 and 100 may be needed toattain the desired crystalline structures described above. During theannealing process, a BCC lattice structure will form at the interface ofthe magnetic free layers 1 and 102 with the junction layers 2 and 103and the magnetic reference layers 3 and 104 with the junction layers 2and 103 in both the elements 10 and 100 because each of the magneticlayers 1, 3, 102, and 104 is made of a CoFe based alloy or aboron-depleted CoFeB alloy. The MgO junction layers 2 and 103 will forma cubic structure that matches the BCC structure at the junctioninterfaces of the magnetic free layers 1, 102 and the magnetic referencelayers 3, 104 in the elements 10 and 100. The magnetic free layers 1 and102, as well as the magnetic reference layers 3 and 104 gain TMR signaland perpendicular magnetic anisotropy from this BCC interfacecrystalline structure matching with the MgO tunnel junction layers 2 and103. Hence, it is desirable to have a MgO tunnel layer with a highdegree of crystallinity to interface with the magnetic reference andfree layers made of CoFe based alloys with high degrees of BCCcrystallinity.

The STTMRAM element 100 also includes the magnetic top reference layer106 and the tantalum layer 105. The magnetic top reference layer 106couples to the magnetic interface reference layer 104 by magneto-staticcoupling or magnetic exchange couple through the Ta layer 105. Thus, themagnetizations 141 and 161 of the magnetic interface reference layer 104and the magnetic top reference layer 106, respectively, are alwaysaligned in parallel. The Ru layer 107 is disposed next to the magnetictop reference layer 106, opposite the Ta layer 105. The magnetic pinnedlayer 108 is disposed next to the Ru layer 107. The magnetizationdirection 181 of the magnetic pinned layer 108 is opposing themagnetization directions 141 and 161 of the magnetic reference layers104 and 106. The magnetic pinned layer 108 exchange couples to themagnetic top reference layer 106 by anti-ferromagnetic-exchange couplinggenerated through the Ru layer 107. Thus, the magnetic pinned layer 108helps maintain an anti-parallel orientation of the magnetization 161 tomagnetization 181. The magneto-static field generated by the magneticpinned layer 108 in the magnetic free layer 102 partially or completelycancels the effective magnetic field generated by the magnetic referencelayers 104 and 106 in the magnetic free layer 102. To attain a highperpendicular anisotropy, the magnetic top reference layer 106 and themagnetic pinned layer 108 of the element 100 are made of super-latticestructures, such as Co/Pt, Co/Pd and Co/Ni, where the super-latticestructure has either a FCC structure or a crystalline structure that isdifferent from the BCC structure of the magnetic interface referencelayer 104 and the magnetic free layer 102. Therefore, after annealing,if the annealing temperature is sufficiently high and the annealing timeis sufficiently long, the non-BCC lattice structure of the magnetic topreference layer 106 and the magnetic pinned layer 108 may propagate toaffect the crystal structures of the magnetic interface reference layer104, the tunnel junction layer 103, and even the magnetic free layer102, thereby degrading the BCC crystalline structures and correspondingperpendicular anisotropy of the magnetic reference layer 104 and themagnetic free layer 102, and resulting in lower final TMR value for theSTTMRAM element 100. Moreover, non-magnetic elements of thesuper-lattice, such as Pt, Pd, and Ni, may also migrate or diffuse intothe junction layer 103 and further reduce the perpendicular anisotropiesof the magnetic interface reference layers 104 and the magnetic freelayer 102, which further reduces the TMR value of the memory element100. While it is possible to use the Ta layer 105 as a diffusion barrierfor the above mentioned non-magnetic elements and as a barrier againstpropagation of non-BCC crystalline structures of the magnetic topreference layer 106 and the magnetic pinned layer 108, the thickness ofthe Ta layer 105 is very thin, generally less than 1 nm, because of theneed for the magnetic top reference layer 106 coupling to the magneticinterface reference layer 104. Under conditions of sufficiently highannealing temperature and/or sufficiently long annealing time, thebarrier properties of the thin Ta layer 105 may be degraded and theperpendicular anisotropies of the magnetic interface reference layer 104and the magnetic free layer 102 will be reduced, resulting in lower TMRfor the memory element 100. Foundry operations at backend of the line(BEOL) may see temperatures as high as 400° C. Hence, it is desirable tohave a thermally stable memory element so that perpendicularanisotropies of magnetic layers and TMR thereof do not get undesirablyaffected and reduced. The MTJ element of an STTMRAM device needs tosurvive such a temperature range.

FIG. 14 shows exemplary normalized TMR vs annealing temperature trends201 and 202 for the memory elements 10 and 100, respectively. Asannealing temperature rises, the TMR for the memory element 10 issignificantly reduced at a temperature of 400° C. while the TMR for thememory element 100 is reduced at as low as 300° C. owing to theexistence of non-BCC structures of the magnetic top reference layer 106and the magnetic pinned layer 108.

Thus, it would be desirable to have an MTJ element structure for STTMRAMapplication in which the magnetic fields exerted by the magnetic pinnedlayer and magnetic reference layer on the free layer effectively canceleach other. It is also advantageous to have an MTJ element structure inwhich FCC or other non-BCC crystalline structures of magnetic layers inthe reference or pinned layer do not reduce the perpendicular anisotropyof the free layer or other magnetic layers after high temperatureannealing treatment, thereby preventing the degradation of the TMR ofthe MTJ element.

FIG. 15 shows an embodiment of the present invention as applied to anMTJ element 500 of an STT-MRAM device. The MTJ element 500 is similar tothe MTJ element 30 of FIG. 4. The MTJ element 500 inherits all thebenefits and advantages of the MTJ element 30 of FIG. 4. With continuingreference to FIG. 15, the MTJ stack is disposed between a top electrode508 and a bottom electrode 509, which are connected to a bit line (notshown) and a selection transistor (not shown), respectively, of theSTTMRAM device. The MTJ stack comprises a perpendicular magneticreference layer 510 formed on top of the bottom electrode 509, aninsulating tunnel junction layer 502 formed on top of the magneticreference layer 510, a perpendicular magnetic free layer 501 formed ontop of the junction layer 502, a non-magnetic tuning layer 507 formed ontop of the perpendicular magnetic free layer 501, and a perpendicularmagnetic pinned layer 505 formed on top of the non-magnetic tuning layer507 and capped by the top electrode layer 508. The magnetic referencelayer 510 further comprises a first reference sublayer 503 and a secondreference sublayer 504 with a non-magnetic coupling layer 506 interposedtherebetween. The second reference sublayer 504 has a magnetization5041; the first reference sublayer 503 has a magnetization 5031; theperpendicular magnetic free layer 501 has a magnetization 5011; and theperpendicular magnetic pinned layer 505 has a magnetization 5051,wherein the magnetizations 5041, 5031, 5011 and 5051 are all orientedperpendicularly to the film plane. The magnetization 5011 of theperpendicular magnetic free layer 501 can be switched into one of thetwo orientations along the direction perpendicular to the film planewith the application of an electric current flowing through the MTJstack between the top electrode 508 and bottom electrode 509 during theoperation of the STTMRAM device. The magnetization 5051 of the magneticpinned layer 505 and the magnetizations 5031 and 5041 of the first andsecond reference sublayers 503 and 504, respectively, are fixed duringthe operation of the STTMRAM device. The magnetization 5051 of themagnetic pinned layer 505 is opposing the magnetizations 5031 and 5041of the first and second reference sublayers 503 and 504, respectively.

The magnetic reference layer 510 of the MTJ element 500 functionssimilarly as the reference layer 23 of the MTJ element 30 in FIG. 4. Themagnetic pinned layer 505 of the MTJ element 500 functions similarly asthe fixed layer 25 of the MTJ element 30. The magnetic free layer 501 ofthe MTJ element 500 functions similarly as the free layer 21 of the MTJelement 30. The magnetic pinned layer 505 and the magnetic referencelayer 510 have different switching fields, similar to the fixed layer 25and the reference layer 23 shown in FIG. 12 to enable antiparallelorientations of the magnetization 5051 and the magnetizations 5031 and5041 of the magnetic reference layer 510 as shown in FIG. 15 by anexternal field setting process similar to that illustrated and describedin FIG. 12. In the MTJ element 500, the first and second referencesublayers 503 and 504 are exchange coupled or magneto-static coupledthrough the non-magnetic coupling layer 506, and their magnetizations5031 and 5041 are aligned in the same direction during the operation ofthe MTJ element 500. Therefore, the coupling layer 506 and the first andsecond reference sublayers 503 and 504 can be combined and regarded as asingle reference layer 510 with a single magnetization. Similar to thefixed layer 25 and the reference layer 23 of the MTJ element 30 in FIG.4, the magnetic pinned layer 505 and the magnetic reference layer 510have different coercivity fields, or switching fields, so that theanti-parallel orientation between the magnetization 5051 andmagnetizations 5031 and 5041 as shown in FIG. 15 can be attained byfollowing the field initialization process illustrated and described inFIG. 12. With the pinned layer 505 having a different switching fieldthan the magnetic reference layer 510, a magnetic field applied to theMTJ element 500 in the perpendicular direction can orient themagnetization of one of the magnetic pinned layer 505 and the magneticreference layer 510 to a direction that is opposite and antiparallel tothe other layer without affecting the magnetization orientation of theother layer.

The function of the non-magnetic tuning layer 507 in the MTJ element 500is to adjust the offset field in the magnetic free layer 501 to as closeto zero as possible. The magnetization 5051 of the magnetic pinner layer505 and the magnetizations 5031 and 5041 of the magnetic reference layer510 generate magnetic fields (mainly magneto-static fields) in themagnetic free layer 501 in the perpendicular direction. With theanti-parallel orientation between the magnetization 5051 and themagnetizations 5031 and 5041, the magnetic fields produced in the freelayer 501 by the magnetic pinned layer 505 and the magnetic referencelayer 510 may cancel each other. When such field cancellation is notperfect, a net external field will exist in the magnetic free layer 501and is called an offset field. The offset field causes the magnetic freelayer 501 to exhibit behavior of asymmetric switching by magnetic fieldor electric current, which is not desirable for the STTMRAM application.The thickness of the tuning layer 507 can be adjusted to control themagnetic field exerted on the magnetic free layer 501 by the magneticpinned layer 505, resulting in close to zero offset field in themagnetic free layer 501.

FIG. 16A shows the normalized offset field in the magnetic free layer501 as a function of the tuning layer thickness for various values of“magnetization thickness product” 1611-1613. In FIG. 16A, the magneticpinned layer 505 has a constant “magnetization thickness product”, i.e.Mst, which is a quantitative measurement of the effective magneticmoment of a magnetic layer and is a product of the effectivemagnetization of a magnetic layer and the thickness of the same magneticlayer. The magnetic reference layer 510 has varying Mst 1611-1613, whichcan occur when the effective Ms of the magnetic reference layer 510changes and/or the thicknesses of the magnetic layers 503 and 504 of thereference layer 510 change. The effective offset field of the magneticfree layer 501 is plotted against the thickness of the tuning layer 507for three different Mst values of the reference layer 510 as illustratedby the curves 1611-1613. The three curves 1611-1613 cross the zerooffset field level at the tuning layer thickness of t11, t12 and t13,respectively. The Mst values of the magnetic reference layer 510corresponding to the curves 1611 and 1613 are highest and lowest,respectively, with the Mst value corresponding to the curve 1612 inbetween. FIG. 16A shows that with the Mst value of the pinned layer 505being constant, a higher Mst value of the magnetic reference layer 510requires a thinner thickness of the tuning layer 507 to maintain anabout zero offset field in the magnetic free layer 501 with t11 beingsmaller than t12 and t12 being smaller than t13.

FIG. 16B shows similar plots as that of FIG. 16A, only that the magneticreference layer 510 now has a constant Mst value, while the magneticpinned layer 505 has varying Mst values, which can occur when theeffective Ms of the magnetic pinned layer 505 changes and/or thethickness of the same layer 505 changes. The effective offset field ofthe magnetic free layer 501 is plotted as a function of the thickness ofthe tuning layer 507 for three different Mst values of the magneticpinned layer 505 as represented by curves 1621-1623. The three curves1621, 1622, and 1623 cross the zero offset field level at tuning layerthickness of t21, t22 and t23, respectively. The Mst values of themagnetic pinned layer 505 corresponding to the curves 1621 and 1623 arelowest and highest, respectively, with the Mst value corresponding tothe curve 1622 in between. FIG. 16B shows that with the Mst value of themagnetic reference layer 510 being constant, a higher Mst value of themagnetic pinned layer 505 requires a thicker thickness of the tuninglayer 507 to achieve close to zero offset field in the free layer 501with t21 being smaller than t22 and t22 being smaller than t23.

Referring now to FIG. 15, the magnetic free layer 501 preferablycomprises a CoFeB alloy and has a BCC lattice structure according to anembodiment of the present invention. The Fe content of magnetic freelayer 501 is preferably 40 atomic percent (at. %) or more and morepreferably 60 at.% or more. The B content is preferably 30 at. % or lessand more preferably 20 at. % or less.

FIG. 17 shows the magnetic free layer 501 may further comprise a freetop sublayer 5014 and a free interface sublayer 5012 formed on top ofthe junction layer 502 according to an embodiment of the presentinvention. The free interface sublayer 5012 may comprise a CoFe, Fe, orCoFeB based alloy with a B content of 10 at. % or less. This freeinterface sublayer 5012 can be deposited as a thin individual layer withthickness of less than 1.0 nm. This free interface sublayer 5012 canalso be the result of annealing the MTJ element 500 to deplete the Bcontent at the interface of the magnetic free layer 501 with thejunction layer 502. The free interface sublayer 5012 can function as aspin-polarization enhancement layer that helps to increase the TMR ofthe MTJ element 500 through the junction layer 502. This free interfacesublayer 5012 can also serve as a perpendicular anisotropy enhancementlayer for the magnetic free layer 501, the perpendicular anisotropy ofwhich is partially or entirely produced by the interface between themagnetic free layer 501 and the junction layer 502.

In another embodiment of the present invention, the magnetic free layer501 may comprise a free bottom sublayer 5015 and a free interfacesublayer 5013 disposed next to the tuning layer 507 as illustrated inFIG. 18. The free interface sublayer 5013 may comprise a CoFe, Fe, orCoFeB based alloy with a B content of 10 at. % or less. This freeinterface sublayer 5013 can also include at least one of the followingelements: Ta, Hf, Zr, and V. The free interface sublayer 5013 can bedirectly deposited as a thin individual layer with thickness of lessthan 1.0 nanometer or can be the result of annealing the MTJ element 500to deplete the B content at the interface of the magnetic free layer 501with the tuning layer 507. The free interface sublayer 5013 can alsoserve as a perpendicular anisotropy enhancement layer for the free layer501, the perpendicular anisotropy of which is at least partiallyproduced by the interface between the magnetic free layer 501 and thetuning layer 507. In still another embodiment of the present invention,the magnetic free layer 501 may comprise a free bottom sublayer 5018 anda free top sublayer 5017 with a free layer insertion sublayer 5016interposed therebetween, as illustrated in FIG. 19. The free layerinsertion sublayer 5016 may comprise a metal or alloy made of Ta,CoFeTa, CoFeBTa, Hf, CoFeHf, CoFeBHf, Zr, CoFeZr, CoFeBZr, V, CoFeV, orCoFeBV. The free layer insertion sublayer 5016 may be directly depositedas a thin individual layer with thickness of less than 1.0 nm and allowsexchange coupling of the free bottom sublayer 5018 to the free topsublayer 5017. Because of the thin thickness of the free layer insertionsublayer 5016, the free bottom sublayer 5018 exchange coupled to thefree top sublayer 5017 behave like a single free layer 501 duringoperation as the magnetizations of the same layers 5017 and 5018 alwaysswitch together. The free layer insertion sublayer 5016 can also act asa perpendicular anisotropy enhancement layer (PEL) for the free layer501 to enhance the surface perpendicular anisotropy of the free layer501 produced by the interface of the free layer 501 with the junctionlayer 502 and by the interface of the free layer 501 with the tuninglayer 507.

Now, referring back to FIG. 15, the insulating junction layer 502 is atunnel junction layer made of magnesium oxide (MgO) with a cubic latticestructure according to an embodiment of the present invention. In someembodiments, the junction layer 502 may comprise at least one of thefollowing oxide materials: magnesium oxide, aluminum oxide, titaniumoxide, and zinc oxide.

The first reference sublayers 503 is substantially similar to themagnetic free layer 501 in structure and composition and is preferablymade of a CoFeB or CoFeB based alloy having a BCC lattice structureaccording to an embodiment of the present invention. The Fe content ispreferably 40 at. % or higher and more preferably 60 at. % or higher.The B content is preferably 30 at. % or lower and more preferably 20 at.% or lower.

According to another embodiment, the first reference sublayer 503 mayfurther comprise a first reference bottom sublayer 5034 and a firstreference interface sublayer 5032 formed thereabove as illustrated inFIG. 20. The first reference interface sublayer 5032 is formed adjacentto the junction layer 502 and may comprise a metal or alloy made ofCoFe, Fe, or CoFeB with a B content of 10 at. % or lower. The firstreference interface sublayer 5032 can be directly deposited as a thinindividual layer with thickness of less than 1.0 nanometer or can alsobe the result of annealing the MTJ element 500 to deplete the B contentat the interface of the first reference sublayer 503 with the junctionlayer 502. The first reference interface sublayer 5032 can function as aspin-polarization enhancement layer that helps to increase the TMR ofthe MTJ element 500 through the junction layer 502. The first referenceinterface sublayer 5032 can also serve as perpendicular anisotropyenhancement layer for the first reference sublayer 503, theperpendicular anisotropy of which is partially or entirely produced bythe interface between the first reference sublayer 503 and junctionlayer 502.

According to still another embodiment, the first reference sublayer 503may comprise a first reference interface sublayer 5033 and a firstreference top sublayer 5035 formed thereabove, as illustrated in FIG.21. The first reference interface sublayer 5033 is formed adjacent tothe coupling layer 506 and may comprise a magnetic metal or alloy madeof CoFe, Fe, or CoFeB with a B content of 10 at. % or lower. The firstreference interface sublayer 5033 may also contain at least one of thefollowing elements: Ta, Hf, Zr, and V. The first reference interfacesublayer 5033 can be directly deposited as a thin individual layer withthickness of less than 1.0 nanometer or can also be the result ofannealing the MTJ element 500 to deplete the B content at the interfaceof the first reference sublayer 503 with the coupling layer 506. Thefirst reference interface sublayer 5033 can also serve as aperpendicular anisotropy enhancement layer for the first referencesublayer 503, the perpendicular anisotropy of which is at leastpartially produced by the interface between the first reference sublayer503 and the coupling layer 506.

According to yet another embodiment of the present invention, the firstreference sublayer 503 may comprise a first reference bottom sublayer5038 and a first reference top sublayer 5037 with a first referenceinsertion sublayer 5036 interposed therebetween, as illustrated in FIG.22. The first reference insertion sublayer 5036 may comprise a metal oralloy made of Ta, CoFeTa, CoFeBTa, Hf, CoFeHf, CoFeBHf, Zr, CoFeZr,CoFeBZr, V, CoFeV, or CoFeBV. The first reference insertion sublayer5036 may be deposited as a thin individual layer with thickness of lessthan 1.0 nanometer and allows direct exchange coupling of the firstreference bottom sublayer 5038 to the first reference top sublayer 5037therethrough. Because of thin thickness of the layer 5036, the firstreference bottom sublayer 5038 exchange coupled to the first referencetop sublayer 5037 behave like a single first reference sublayer 503during operation and the magnetizations of the same layers 5037 and 5038always switch together. The first reference insertion sublayer 5036 canalso act as a perpendicular anisotropy enhancement layer (PEL) for thefirst reference sublayer 503 to enhance the surface perpendicularanisotropy of the same layer 503 produced by the interface of the firstreference sublayer 503 with the junction layer 502 and by the interfaceof the first reference sublayer 503 with the coupling layer 507.

Referring back to FIG. 15, the magnetic pinned layer 505 may comprise ahigh perpendicular anisotropy material such that its switching field issufficiently high that during operation of the MTJ element 500, themagnetization 5051 of the magnetic pinned layer 505 is fixed. Themagnetization 5051 can be switched by an externally applied strongmagnetic field that is higher than the switching field of the magneticpinned layer 505. The high perpendicular anisotropy of the magneticpinned layer 505 is generated by the crystalline or lattice structurethereof. In an embodiment of the present invention, the magnetic pinnedlayer 505 may be made of FePt, CoCrPt, or a material comprising Co, Fe,or Ni, and at least one element from the group of platinum (Pt),chromium (Cr), and palladium (Pd). In another embodiment, the magneticpinned layer 505 may comprise a supper-lattice structure formed byrepeated interlacing of a magnetic layer and a weak magnetic ornon-magnetic layer, in which the magnetic layer is made of Co, Fe, orCoFe and the weak magnetic or non-magnetic layer is made of Pd, Pt, orNi. Super-lattice structures may be made by repeated interlacing of basepairs of Co/Pt, Co/Pd, Co/Ni, CoFe/Pt, CoFe/Pd, and CoFe/Ni, where ahybridization of different base pairs can also be employed. Most knownmaterials and structures with high anisotropy for the magnetic pinnedlayer 505, as discussed above, have crystalline or lattice structuresthat are different from the BCC lattice structure of the magnetic freelayer 501 at the interface next to the junction layer 502 and the cubiclattice structure of the junction layer 502. The lattice and crystallinestructure of the magnetic pinned layer 505 being different from BCCmakes it necessary to isolate the lattice structure of the magneticpinned layer 505 from the lattice structures of the magnetic free layer501 and the junction layer 502. Lattice mismatch or interference betweenthe magnetic pinned layer 505 and the magnetic free layer 501 and thejunction layer 502 will lead to degradation of the junction layer 502(when FCC or other lattice structure of the magnetic pinned layer 505propagates to the magnetic free layer 501 and the junction layer 502during annealing), thereby lowering the TMR, and/or degradation of theperpendicular anisotropy of the magnetic pinned layer 505 (when thelattice structures of the magnetic free layer 501 and the junction layer502 propagate to affect the FCC or other lattice structure of themagnetic pinned layer 505). Additionally, the magnetic pinned layer 505may contain elements, such as Pd, Pt, Ni, and Cr, that may undesirablydiffused into the junction layer 502 or the magnetic free layer 501, asthe presence of these elements will reduce the TMR of the MTJ element500. Hence, in addition to controlling the offset field in the magneticfree layer 501, the tuning layer 507 needs to function as a diffusionbarrier and a lattice interference barrier to prevent element diffusionand lattice interference between the magnetic pinned layer 505 and thelayers of the magnetic free layer 502 and the junction layer 502.

The tuning layer 507 is preferably non-magnetic or is substantiallynon-magnetic with a thickness larger than 1 nm, and most preferably witha thickness in the range of about 2 nm to about 10 nm. In an embodimentof the present invention, the tuning layer 507 may comprise anon-magnetic conductive material made of Ta, Ti, TaN, TiN, W, or anycombination thereof. The tuning layer 507 is preferably made of Ta witha thickness range of about 2 nm to about 5 nm. The Ta layer promotesperpendicular anisotropy of the magnetic free layer 501 generated fromthe interface of the magnetic free layer 501 and the junction layer 502by enhancing B depletion from the magnetic free layer 501 interface withthe junctions layer 502, thereby forming a higher degree of BCC latticeat the interface of the magnetic free layer 501 with the junction layer502 while preventing element diffusion and lattice interference betweenthe magnetic pinned layer 505 and the layers of the magnetic free layer502 and the junction layer 502.

In some embodiments, the tuning layer 507 may also include anon-magnetic seed layer (not shown in FIG. 15) formed adjacent to themagnetic pinned layer 505 for enhancing epitaxial growth of the magneticpinned layer 505 into a preferred lattice or crystalline structure witha high perpendicular anisotropy. The non-magnetic seed layer may be madeof Ru, Cu, TiCr, Cr, Al, or any combination thereof. In some embodimentswhere the magnetic pinned layer 505 is made of Co/Ni super-lattice, thetuning layer 507 preferably comprises a TaN diffusion barrier layerhaving a thickness range of about 2 nm to about 4 nm and a Cu seed layerwith a thickness range of about 1 nm to about 2 nm formed thereabove.

In another embodiment, the tuning layer 507 may have a multi-layerstructure that includes a thin oxide layer with a thickness of less than2 nm at the interface next to the magnetic free layer 501 and aconductive upper layer formed adjacent to the magnetic pinned layer 505.The resistivity of the oxide layer is lower than the resistivity of thejunction layer 502 and no TMR signal is produced from this oxide layer.A multi-layer example is the MgO/Ta structure, where the MgO layer (notshown in FIG. 15) with a thickness range of about 0.5 nm to about 1.1 nmis formed adjacent to the magnetic free layer 501 and the Ta layer witha thickness of about 2 nm or thicker formed adjacent to the magneticpinned layer 505. The MgO layer can enhance the perpendicular anisotropywithin the magnetic free layer 501 at interface thereof with the MgOlayer.

Alternatively, the tuning layer 507 may also have a multi-layerstructure of MgO/CoFeB/Ta, where the MgO is formed adjacent to themagnetic free layer 501 and has a thickness of about 0.5 nm to about 1.1nm, the CoFeB layer has a thickness of about 0.2 nm to about 0.6 nm, andthe Ta layer is formed next to the magnetic pinned layer 505 and has athickness of about 2 nm or thicker. The CoFeB layer is magnetically“dead” and does not produce discernible magnetic moment at thicknessrange thereof. The purpose of the CoFeB layer in the tuning layer 507 isto enhance the formation of BCC structure in the magnetic free layer 501through the MgO layer of the tuning layer 507, thereby achieving higherperpendicular anisotropy in the magnetic free layer 501.

The second reference sublayer 504 of FIG. 15 is similar to the magneticpinned layer 505 in composition and structure. The second referencesublayer 504 is preferably formed of a thermally stable material suchthat elements thereof do not diffuse into the first reference sublayerlayer 503 and the junction layer 502. It is also desirable that thenon-BCC structure of the second reference sublayer 504 does notpropagate to affect the lattice structures of the first referencesublayer layer 503 and the junction layer 502.

Like the tuning layer 507, the coupling layer 506 is a non-magneticlayer which may function as a diffusion barrier for preventinginter-diffusion between the second reference sublayer 504 and the layersof the first reference sublayer 503 and the junction layer 502. Thecoupling layer 506 may also prevent the lattice interference or mismatchbetween the second reference sublayer 504 and the layers of the firstreference sublayer 503 and the junction layer 502. However, unlike thetuning layer 507, the coupling layer 506 is sufficiently thin to providestrong enough magnetic coupling between the second reference sublayer504 and the first reference sublayer 503. Thus, the thickness of thecoupling layer 506 is preferably thinner than that of the tuning layer507.

In an embodiment of the present invention, the coupling layer 506 may bea non-magnetic conductive layer made of Ta, Ti, TaN, TiN, W, or anycombination thereof. The non-magnetic coupling layer 506 is preferablymade of Ta with a thickness range of about 0.3 nm to about 0.8 nm. Thesecond reference sublayer 504 and the first reference sublayer 503couples to each other by direct magnetic exchange coupling through thecoupling layer 506 owing to thin thickness thereof. Alternatively, thecoupling layer 506 may be a Ta layer with a thickness range of about 2nm or thicker. At this thickness range, the second reference sublayer504 and the first reference sublayer 503 couple to each other throughmagneto-static fields. The coupling layer 506 made of a thicker Tapromotes the perpendicular anisotropy of the first reference sublayer503 generated from the interface of the first reference sublayer 503 andthe junction layer 502 by enhancing B depletion from the interface ofthe first reference sublayer 503 with the junctions layer 502, therebyforming a higher degree of BCC lattice at the interface of the firstreference sublayer 503 with the junction layer 502 while preventingelement inter-diffusion and lattice interference or mismatch between thesecond reference sublayer 504 and the layers of the first referencesublayer 503 and the junction layer 502.

In another embodiment, the coupling layer 506 may have a multi-layerstructure that includes a thin oxide layer with a thickness of less thanabout 2 nm formed at the interface adjacent to the first referencesublayer 503 and a conductive bottom layer formed adjacent to the secondreference sublayer 504. The electrical resistivity of the oxide layer islower than that of the junction layer 502 and no TMR signal is producedfrom this oxide layer when a spin polarization current is applied to theMTJ element 500. A multi-layer example of the coupling layer 506 is aTa/MgO structure, where the MgO layer (not shown) with a thickness rangeof about 0.5 nm to about 1.1 nm is formed adjacent to the firstreference sublayer 503 and the Ta layer thickness is in the range ofabout 1 nm or thicker. The MgO layer can enhance the perpendicularanisotropy within the first reference sublayer 503 at interface thereofwith the MgO layer.

Alternatively, the multi-layer structure of the coupling layer 506 mayhave a Ta/CoFeB/MgO structure, in which the MgO with a thickness in therange of about 0.5 nm to about 1.1 nm is an interface layer formedadjacent to the first reference sublayer 503; the CoFeB layer has athickness range of about 0.2 nm to about 0.6 nm; and the Ta layer with athickness range of about 1 nm or less. The CoFeB layer is magnetically“dead” and does not produce discernible magnetic moment at thicknessrange thereof. The purpose of the CoFeB layer in the coupling layer 506is to enhance the formation of BCC structure in the first referencesublayer 503 through the MgO layer of the coupling layer 506, therebyachieving higher perpendicular anisotropy in the first referencesublayer 503.

The top electrode 508 and the bottom electrode 509 each comprises one ormore of the following conductive materials: Ta, TaN, TiN, W, and Cu.Below are some examples of the MTJ stack of the MTJ element 500 inbetween the top electrode 508 and the bottom electrode 509:

Example 1

the second reference sublayer 504 formed of repeated base pairs ofCo/Pt, or Co/Pd, or Co/Ni super-lattice structure with the FCC lattice;

the coupling layer 506 formed of Ta with a thickness range of about 0.3nm to about 0.8 nm;

the first reference sublayer 503 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the junction layer 502 formed of MgO with a cubic lattice structure;

the magnetic free layer 501 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the tuning layer 507 formed of Ta with a thickness range of about 2 nmor thicker; and

the magnetic pinned layer 505 formed of repeated base pairs of Co/Pt, orCo/Pd, or Co/Ni super-lattice structure with the FCC lattice.

Example 2

the second reference sublayer 504 formed of repeated base pairs ofCo/Pt, or Co/Pd, or Co/Ni super-lattice structure with the FCC lattice;

the coupling layer 506 formed of Ta with a thickness of about 2 nm orthicker;

the first reference sublayer 503 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the junction layer 502 formed of MgO with a cubic lattice structure;

the magnetic free layer 501 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the tuning layer 507 formed of Ta with a thickness range of about 2 nmor thicker; and

the magnetic pinned layer 505 formed of repeated base pairs of Co/Pt, orCo/Pd, or Co/Ni super-lattice structure with the FCC lattice.

Example 3

the second reference sublayer 504 formed of repeated base pairs ofCo/Pt, or Co/Pd, or Co/Ni super-lattice structure with the FCC lattice;

the coupling layer 506 formed of Ta with a thickness range of about 0.3nm to about 0.8 nm;

the first reference sublayer 503 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the junction layer 502 formed of MgO with a cubic lattice structure;

the magnetic free layer 501 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the tuning layer 507 having a MgO/CoFeB/Ta structure, where the MgOlayer with a thickness range of about 0.5 nm to about 1.1 nm is formedadjacent to the magnetic free layer 501; the CoFeB layer has a thicknessrange of about 0.2 nm to 0.6 nm; and the Ta layer with a thickness rangeof about 2 nm to about 3 nm is formed adjacent to the magnetic pinnedlayer 505; and

the magnetic pinned layer 505 formed of repeated base pairs of Co/Pt, orCo/Pd, or Co/Ni super-lattice structure with the FCC lattice.

Example 4

the second reference sublayer 504 formed of repeated base pairs ofCo/Pt, or Co/Pd, or Co/Ni super-lattice structure with the FCC lattice;

the coupling layer 506 formed of Ta with a thickness of about 2 nm orthicker;

the first reference sublayer 503 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the junction layer 502 formed of MgO with a cubic lattice structure;

the magnetic free layer 501 formed of a CoFeB alloy and having aninterface layer with the BCC lattice structure next to the junctionlayer 502;

the tuning layer 507 having a MgO/CoFeB/Ta structure, where the MgOlayer with a thickness range of about 0.5 nm to about 1.1 nm is formedadjacent to the magnetic free layer 501; the CoFeB layer has a thicknessrange of about 0.2 nm to 0.6 nm; and the Ta layer with a thickness rangeof about 2 nm to about 3 nm is formed adjacent to the magnetic pinnedlayer 505; and

the magnetic pinned layer 505 formed of repeated base pairs of Co/Pt, orCo/Pd, or Co/Ni super-lattice structure with the FCC lattice.

Referring now to FIG. 23, which shows the normalized TMR as a functionof the annealing temperature for the MTJ elements 10, 100, and 500. Thecurve 5002 corresponds to the MTJ element 500 with the above-describedExample 1 configuration, while curves 201 and 202 corresponds the MTJelements 10 and 100, respectively. As the annealing temperature reaches400° C., the TMR values of the MTJ elements 10 and 100 markedly decreaseowing to crystalline mismatch or inter-diffusion of elements from thepinned layer 108 and the reference layer 106 with the FCC lattice andPt, Pd or Ni constituent to the BCC lattice structure around thejunction layer 103. The TMR value of the MTJ element 500, however, stillmaintains at a high level even at 400° C. The MTJ element 500 has betterthermal stability than the elements 10 and 100 because the tuning layer507 and the coupling layer 506 can provide effective diffusion barrierand prevent the non-BCC structures from propagating to the magneticlayers adjacent to the junction layer 502.

The film stacking sequence of the MTJ element 500 illustrated in FIG. 15can be reversed without affecting performance thereof. FIG. 24 shows theMTJ element 600 having the same film layers as the MTJ element 500 butwith a reversed film stacking sequence. Accordingly, the element 600comprises a magnetic pinned layer 605 formed on top of a bottomelectrode 609, non-magnetic tuning layer 607 formed on top of themagnetic pinned layer 605, a magnetic free layer 601 formed on top ofthe non-magnetic tuning layer 607, an insulating tunnel junction layer602 formed on top of the magnetic free layer 601, a magnetic referencelayer 610 formed on top of the tunnel junction layer 602 and capped by atop electrode 608. The magnetic reference layer 610 further comprises afirst reference sublayer 603 and a second reference sublayer 604 with anon-magnetic coupling layer 606 interposed therebetween. Each of thelayers 601-610 has the same ranges of physical properties, structuralcompositions, and magnetic properties as the corresponding layer in theelement 500 of FIG. 15. In other words, the element 600 is the element500 placed upside down.

The second reference sublayer 604 has a magnetization 6041; the firstreference sublayer 603 has a magnetization 6031; the perpendicularmagnetic free layer 601 has a magnetization 6011; and the perpendicularmagnetic pinned layer 605 has a magnetization 6051, wherein themagnetizations 6041, 6031, 6011 and 6051 are all orientedperpendicularly to the film plane. The magnetization 6011 of theperpendicular magnetic free layer 601 can be switched into one of thetwo orientations along the direction perpendicular to the film plan withthe application of an electric current flowing through the MTJ stackbetween the top electrode 608 and bottom electrode 609 during theoperation of the STTMRAM device. The magnetization 6051 of the magneticpinned layer 605 and the magnetizations 6031 and 6041 of the first andsecond reference sublayers 603 and 604, respectively, are fixed duringthe operation of the STTMRAM device. The magnetization 6051 of themagnetic pinned layer 605 is opposing the magnetizations 6031 and 6041of the first and second reference sublayers 603 and 604, respectively.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall such alterations and modification as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A spin transfer torque magnetic random accessmemory (STTMRAM) element comprising: a magnetic pinned layer having afirst fixed magnetization direction substantially perpendicular to alayer plane thereof; a magnetic free layer separated from said magneticpinned layer by a tuning layer and having a variable magnetizationdirection substantially perpendicular to a layer plane thereof; and amagnetic reference layer separated from said magnetic free layer by aninsulating tunnel junction layer and having a second fixed magnetizationdirection substantially opposite to said first fixed magnetizationdirection provided in said magnetic pinned layer, wherein said tuninglayer comprises at least one insulating layer and at least oneconductive layer.
 2. The STTMRAM element of claim 1, wherein said atleast one insulating layer is made of magnesium oxide, aluminum oxide,or silicon oxide.
 3. The STTMRAM element of claim 1, wherein said atleast one conductive layer is made of ruthenium, tantalum, copper,silver, or gold.
 4. The STTMRAM element of claim 1, wherein saidmagnetic pinned layer comprises a multilayer structure formed byinterleaving layers of a first material with layers of a secondmaterial, at least one of said first and second materials beingmagnetic.
 5. The STTMRAM element of claim 4, wherein said first materialis made of cobalt, iron, or any combination thereof, said secondmaterial is made of platinum, palladium, nickel, or any combinationthereof.
 6. The STTMRAM element of claim 1, wherein said magnetic freelayer comprises cobalt and iron.
 7. The STTMRAM element of claim 1,wherein said insulating tunnel junction layer comprises magnesium oxide.8. The STTMRAM element of claim 1, wherein said magnetic reference layercomprises cobalt and iron.
 9. A spin transfer torque magnetic randomaccess memory (STTMRAM) element comprising: a magnetic pinned layerhaving a first fixed magnetization direction substantially perpendicularto a layer plane thereof; a magnetic free layer separated from saidmagnetic pinned layer by a tuning layer and having a variablemagnetization direction substantially perpendicular to a layer planethereof; a first magnetic reference layer separated from said magneticfree layer by an insulating tunnel junction layer and having a secondfixed magnetization direction that is substantially perpendicular to alayer plane thereof and is substantially opposite to said first fixedmagnetization direction; and a second magnetic reference layer separatedfrom said first magnetic reference layer by a coupling layer and havingsaid second fixed magnetization direction, wherein said first magneticreference layer further includes at least one interface layer formedadjacent to said insulating tunnel junction layer or said coupling layeror both of said layers.
 10. The STTMRAM element of claim 9, wherein saidfirst magnetic reference layer includes an interface layer made of Fe orCoFe formed adjacent to said insulating tunnel junction layer.
 11. TheSTTMRAM element of claim 9, wherein said first magnetic reference layerincludes an interface layer made of Fe or CoFe formed adjacent to saidcoupling layer.
 12. The STTMRAM element of claim 9, wherein saidmagnetic pinned layer comprises a multilayer structure formed byinterleaving layers of a first material with layers of a secondmaterial, said first material being made of cobalt and second materialbeing made of platinum, palladium, or nickel.
 13. The STTMRAM element ofclaim 9, wherein said tuning layer is non-magnetic.
 14. The STTMRAMelement of claim 9, wherein said tuning layer comprises an insulatinglayer and at least one conductive layer.
 15. The STTMRAM element ofclaim 9, wherein each of said magnetic free layer and said firstmagnetic reference layer comprises cobalt and iron.
 16. The STTMRAMelement of claim 9, wherein said insulating tunnel junction layercomprises magnesium oxide
 17. The STTMRAM element of claim 9, whereinsaid second magnetic reference layer comprises a multilayer structureformed by interleaving layers of a first material with layers of asecond material, at least one of said first and second materials beingmagnetic.
 18. The STTMRAM element of claim 17, wherein said firstmaterial is made of cobalt, iron, or any combination thereof, saidsecond material is made of platinum, palladium, nickel, or anycombination thereof.
 19. A spin transfer torque magnetic random accessmemory (STTMRAM) element comprising: a magnetic pinned layer having afirst fixed magnetization direction substantially perpendicular to alayer plane thereof; a magnetic free layer separated from said magneticpinned layer by a tuning layer and having a variable magnetizationdirection substantially perpendicular to a layer plane thereof; a firstmagnetic reference layer separated from said magnetic free layer by aninsulating tunnel junction layer and having a second fixed magnetizationdirection that is substantially perpendicular to a layer plane thereofand is substantially opposite to said first fixed magnetizationdirection; and a second magnetic reference layer separated from saidfirst magnetic reference layer by a coupling layer and having saidsecond fixed magnetization direction, wherein said second magneticreference layer has a different lattice structure from said insulatingtunnel junction layer.
 20. The STTMRAM element of claim 19, wherein saidsecond magnetic reference layer has a face centered cubic (FCC) latticestructure.